As discharge limits for metals tighten, adsorption processes for high level treatment of metal bearing wastes becomes increasingly attractive. Adsorption is capable of removing many metals over a wider pH range and to much lower levels than precipitation. Additionally, adsorption can often remove complexed metals which would not be checked by conventional treatment processes.
An adsorbent commonly present in metal treatment processes is an amorphous iron oxide called ferrihydrite. A disadvantage of such treatment is that ferrihydrite forms a sludge product from which it is difficult to recover purified water. In order to address this problem, a water treatment product has been described which consists of washed sand coated with ferrihydrite (M Edwards and M M Benjamin, Jnl. Water Poll Control Fed, Vol 61, Part 9, 1989, pages 1523-1533). That product has also been tested for removal of arsenic from drinking water (F G A Vagliasindi et at, Proceedings Water Quality Technology Conference, Part 2, New Orleans, Nov. 12-16 1995, pages 1829-1853).
In Europe and USA the permitted quantities of arsenic in drinking water have been, or will shortly be, reduced from 200 xcexcg/l to 50 xcexcg/l and on to 20 or 10 xcexcg/l. As a water treatment product for removing arsenic, activated alumina has been proposed (Canadian patent 2,090,989). The particles of activated alumina are robust and readily separable from treated water. Although activated alumina is by itself an active adsorbent of arsenic and other heavy metals, there is a need for an even better material. This need has been addressed in WO 96/37438, which proposes water treatment compositions comprising lanthanum oxides and alumina. But lanthanum oxides would be prohibitively expensive for the treatment of very large volumes of water.
According to the present invention there is provided a water treatment product which is a particulate material having a specific surface area of at least 1.0 M2/g, or an artefact formed by bonding together such particulate material, and having an insoluble ferric iron coating. Preferably, the particulate material is porous and may have through pores, closed pores or both. The artefacts formed from the particulate material are typically cylindrical or brick shaped.
The particulate material is preferably non-metallic and a mineral or inorganic material. Preferred materials which may act predominantly as substrates for the ferric iron coating, include Zeolites, Ferrierite, Mordenite, Sodalite, pillared clays and activated clays. Preferred are alumina-based materials including alumina itself and bauxite. The particulate material or the artefact formed therefrom is preferably robust, resistant to crushing, and does not form a fine powder or sludge during use.
The individual particles in the particulate material, which may be accretions of fine particles, need to be sufficiently large to be easily separable from treated water. The individual particles may be as fine as having a mean size of 5 xcexcm or 10 xcexcm, although coarse particles are more readily separated from the treated water. Preferably the individual particles have a mean size of 100 xcexcm to 5000 xcexcm, e.g. from 200 xcexcm-1000 xcexcm. They may be formed by agglomeration or pelleting or crushing.
The particulate material used herein may be of alumina trihydrate as produced by the Bayer process, or calcined alumina. Preferably there is used activated alumina, a product formed by heating alumina trihydrate at 300-800xc2x0 C. Activated alumina has the advantage of a large specific surface area. Thus for example the commercial product AA400G has a specific surface area of 260-380 m2/g. Alternatively, the porous medium may be of bauxite, or other alumina-containing mineral such as zeolite, clay or hydrotalcite. The non-volatile content of bauxite comprises from 40 or 50-95 wt % of alumina together with from 3 or 5-25 wt % of ferric oxide. Activated bauxite is a preferred material which may be formed by heating the mineral at a temperature in the range 300-800xc2x0 C., and may typically have a specific surface area of from 100 or 150-200 m2/g. Because the iron content of bauxite is present in, rather than on, the particle surface, it is generally not counted as part of the insoluble ferric iron coating of this invention.
Particulate materials having a high specific surface area show a high capacity for adsorbing contaminants and removing them from water. The water treatment product of this invention preferably has a specific surface area of 1.0-400 m2/g, e.g. at least 10 m2/g, particularly at least 100 m2/g. 
The particulate material may be provided with a precipitated insoluble ferric iron coating by soaking it in a ferric solution, e.g. an aqueous solution of ferric sulphate or ferric chloride. Then the water is removed by evaporation or otherwise and the product dried at elevated temperature, e.g. of 50-500xc2x0 C. and preferably 50-200xc2x0 C., to convert ferric salts to an insoluble ferric iron coating, probably a hydrated ferric oxide or ferrihydrate. The preparative technique described in the M Edwards reference noted above is suitable. The ferric iron coating may constitute from 0.01% to 50%, preferably 0.1% to 10%, by weight of the water treatment product.
Another way of providing a coated particulate material which has been found particularly suitable for large scale operation is as follows: a suitable grade of activated alumina (such as AA400G28-48 mesh) is saturated in a ferric solution, for example of ferric chloride or preferably ferric sulphate with periodic agitation for up to about 6 hours. Sodium hydroxide solution is added to complete the hydrolysis and form an insoluble hydrated ferric iron oxide coating on the activated alumina using a means such as a pH meter to control the pH to 7.5 to 8. The product is rinsed thoroughly to remove all fine material and dried either at room temperature or at elevated temperature.
An alternative method of making a water treatment product according to the invention comprises treating a ferruginous ore with acid liquid so as to leach out iron from the ore, and then raising the pH of the liquid so as to form a precipitated ferric iron coating on the surface of the ore. For example, the ore may be treated with hydrochloric acid at a pH of around 3, and the pH subsequently raised to about 7 by use of sodium hydroxide. The resulting product is filtered, washed and dried preferably at elevated temperature as before. Also included within the scope of the invention is a water treatment product which is a ferruginous ore having a precipitated ferric iron coating on its surface. Preferably the ore is bauxite particularly activated bauxite.
As demonstrated in the examples below, the water treatment product of this invention has a combination of useful properties: excellent capacity and avidity for rapidly adsorbing inorganic contaminants from water being treated; robust material which is easily separable from treated water and can be treated to recover inorganic contaminants and so permit reuse without losing its structure.
The invention also includes a water treatment method, which method comprises contacting water to be treated with the water treatment product herein described, and thereafter recovering treated water containing a reduced concentration of an organic material or cation or anion particularly at least one heavy metal or As or Se or F. Batch treatment typically involves agitating the water to be treated with an aliquot of the water treatment product, the amount of which is chosen in order to achieve a desired degree of water purification in a desired time, typically less than 1 hour. Continuous methods are also possible as well known in the art.
Optimum conditions for removal of organic materials and of inorganic materials are generally different. Depending on the nature of the contaminant to be removed, it may be advantageous to adjust the pH of the water in order to improve the performance of the water treatment product. Thus for example, arsenic is best removed at a pH of 5 to 7 preferably 5.5, while fluoride is best removed at a pH of 6 to 8 preferably 7.